Method of determining the doping concentration across a surface of a semiconductor material

ABSTRACT

A method ( 100 ) of determining a doping concentration of a semiconductor material ( 101 ) includes the steps of moving carriers ( 102 ) in the material, wherein a number of carriers is a function of the doping concentration of the material ( 101 ). The carriers are deflected ( 130 ) toward a surface ( 110 ) of the material ( 101 ) and an accumulated charge profile on the surface of the material, due to the deflected carriers, is detected ( 140 ) and used to calculate ( 180 ) the doping concentration across a surface ( 110 ) of the material ( 101 ).

FIELD OF THE INVENTION

[0001] The present invention generally relates to a method ofdetermining a doping concentration for a semiconductor startingmaterial, and more particularly relates to a method of scanning asemiconductor wafer and mapping a doping concentration across the wafersurface using the Hall effect.

BACKGROUND OF THE INVENTION

[0002] The fabrication of semiconductor integrated circuits begins witha semiconductor wafer (e.g., silicon) which is often referred to as the“starting material.” Typically the starting material is a lightly dopedp-type wafer (with an approximate dose of about 5×10¹⁴−1×10¹⁵ atoms/cm²)having a <100> crystal orientation. It is desirable that the dopingconcentration (atoms/cm³) be relatively uniform across a single wafer(e.g., within about 1%) and be within a range of about 10¹⁴ to 10¹⁶atoms/cm³ and be uniform from wafer to wafer to provide a resistivity ofabout 2 to 100 Ω-cm. One of the reasons that the doping level uniformityis important is that the doping level of the wafer (i.e., the substratefor various semiconductor devices) impacts the resultant thresholdvoltage for transistors built on and in the substrate as well as othertransistor device parameters.

[0003] When the source and substrate of a transistor are shortedtogether (V_(SB)=0), the transistor threshold voltage (V_(T)) can becharacterized by the following equation:

V _(T) = V _(TO)= φ_(ms)−2φ_(f) −Q _(tot) /C _(OX) −Q _(BO) /C _(OX),

[0004] wherein φ_(ms) is the work function difference between the gatematerial and the bulk silicon in the transistor channel, φ_(f) is theequilibrium electrostatic potential, Q_(BO) is the charge stored perunit area (C/cm²) in the depletion region, C_(OX) is the gate oxidecapacitance per unit area (F/cm²), and Q_(tot) is the total positiveoxide charge per unit area present at the oxide/bulk interface. φ_(ms),φ_(f) and Q_(BO) are each dependent upon the doping concentration of thestarting material, wherein an increase in the doping concentration ofthe substrate changes the above parameters and results in an increase inthe threshold voltage (V_(T)) Consequently, nonuniform dopingconcentrations within a wafer and between wafers is undesirable since itresults in variations in the resulting transistor threshold voltages forvarious semiconductor devices.

[0005] In addition to it being desirable to provide starting materialwhich provides uniform transistor threshold voltages within a wafer andbetween wafers, it is also desirable for the starting material toprovide a low source/drain-to-substrate capacitance, a highsource/drain-to-substrate breakdown voltage, high current mobility and alow sensitivity to source-substrate bias effects. A deviation in thedesired doping concentration, however, impacts the abovecharacteristics. For example, an increase in the wafer dopingconcentration undesirably results in a decrease in the junctionbreakdown voltage, increases the junction capacitance and lowers thecarrier mobility.

[0006] Because the wafer doping concentration level and uniformity is animportant characteristic, electrical specifications are provided for thestarting material which include, for example: the conductivity type ofthe wafer, the average resistivity or resistivity range (Ω-cm), theradial resistivity gradient (% variation) and resistivity variations.The conductivity type information includes whether the wafer is ann-type or a p-type wafer and indicates what element was used to dope thewafer (e.g., arsenic, phosphorous, boron, etc.). The wafer resistivityrelates to the doping density of the wafer (atoms/cm³) and is measuredusing a four-point probe technique. The radial resistivity gradientprovides a measure of the variation of the resistivity between thecenter and selected outer regions of the wafer, and the resistivityvariations represent local variations of the resistivity on the wafer.Both the radial resistivity and the resistivity variations are alsomeasured using a four-point probe technique.

[0007] The four-point probe technique is used to measure the sheetresistance Rs of a film. The sheet resistance Rs of a film (which inthis instance is the wafer) is determined as follows in conjunction withprior art FIG. 1. The resistance (R) of a rectangular shaped film oflength (L), width (W) and thickness (t) is given by the equation:

R=ρL/tW,

[0008] wherein ρ equals the resistivity of the film, which is unique fora given material, and is measured in Ω-cm. If the length L is equal tothe width W, then the rectangle is a square and the equation reduces to:

R=ρ/t=Rs,

[0009] wherein Rs is the sheet resistance in Ω/square and is independentof the size of the square (but does depend on the resistivity of thematerial and the thickness of the film). Therefore the resistivity ρ andthe sheet resistance Rs are distinct parameters that are related by theabove equation.

[0010] The four-point probe method is illustrated in prior art FIG. 2.If the sample film may be approximated as semi-infinite with respect tothe spacings (s) between the four probes (which are spaced apartsubstantially equally from one another), the current (I) is driven asshown and the voltage drop (V₁−V₂) is measured across the remainingprobes as illustrated in prior art FIG. 2. The sheet resistance may thenbe calculated according to the following equation:

Rs=(V ₁ −V ₂)(2πs)/It.

[0011] To prevent erroneous readings using the four point technique(e.g., due to thermoelectric heating and cooling) the measurement isoften performed with current forced in both directions and the tworeadings are averaged. Further, the test is often performed at severalcurrent levels (i.e., I₁, I₂, etc.), until the proper current level isfound. For example, if the current is too low, the forward and reversecurrent readings will substantially differ and if the current is toohigh, I²R heating will result in the measured reading drifting overtime. Although the American Society for Testing and Materials (ASTM)provides standards which recommend current levels for a givenresistivity range, one may still need to vary the current about therecommended value to achieve the optimum current for an accuratemeasurement which undesirably takes extra time.

[0012] The sheet resistance Rs and the resistivity p are found using themeasured results and the equation V/I(2πs), wherein s is the probespacing. The above equation, however, is only accurate if the sample issemi-infinite with respect to the probe spacings, which is often not anaccurate assumption. Thus, the sheet resistance is typically calculatedby the relation:

Rs=(V/I)F₁,

[0013] wherein F₁ is a correction factor which is a function of theaverage probe distance s and the wafer diameter D (i.e., F₁= f(s/D)).

[0014] Since the four-point probe technique uses a correction factor andoccupies a space of at least 3s due to the four probes, the readings arenot totally accurate and further represent merely an average resistivitywithin the region of 3s. Consequently, it would be desirable to have amethod of determining the resistivity of the starting material thatprovides a more accurate and convenient resistivity reading and a higherresolution mapping of the resistivity across the wafer to therebydetermine whether the doping level concentration is sufficientlyuniform.

SUMMARY OF THE INVENTION

[0015] The present invention relates to a method of mapping the dopingconcentration across the surface of a material such as a waferconstituting starting material for semiconductor processing. The methodincludes forcing a current through the wafer and subjecting the wafer toa magnetic field. The magnetic field deflects the carriers in the wafertoward the wafer surface according to the Hall effect. The number ofcarriers deflected to the wafer surface relates to the dopingconcentration at each point in the wafer, thereby resulting in a chargeaccumulation profile at the wafer surface. The charge accumulationprofile is detected and used to calculate the doping concentration ateach point on the wafer surface, thereby resulting in a dopingconcentration mapping of the wafer.

[0016] According to one aspect of the present invention, the methodincludes placing two probes on the surface of the wafer to be measured.A constant current is forced through two probes using a current source(e.g., moves charge carriers. The wafer is subjected to a magnetic fieldwhich is oriented perpendicular to the current path and according tophysical principles characterized as the Hall effect, the magnetic fieldexerts a force on the carriers in the wafer, thereby deflecting thecarriers toward the surface of the wafer. The number of carriersdeflected at each point within the wafer is a function of the dopingconcentration at that point, wherein the greater the dopingconcentration, the greater the number of carriers. The deflectedcarriers reach the wafer surface and form a charge accumulation profileat the wafer surface which reflects the doping concentration in thewafer. The charge accumulation profile is then scanned using a chargedetection device such as an atomic force microscope and the profile isused to either evaluate the doping uniformity across the wafer byanalyzing the profile or to calculate the doping concentration at eachpoint across the wafer.

[0017] To the accomplishment of the foregoing and related ends, theinvention comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,of but a few of the various ways in which the principles of theinvention may be employed. Other objects, advantages and novel featuresof the invention will become apparent from the following detaileddescription of the invention when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a prior art perspective view of a conductive film usedfor illustrating the principle of a film's sheet resistance;

[0019]FIG. 2 is a prior art fragmentary perspective view illustrating afour-point probe technique for measuring a sheet resistance of a film;

[0020]FIG. 3 is a flow chart illustrating a method of determining thedoping concentration of a material across the material surface accordingto the present invention;

[0021]FIG. 4 is a flow chart illustrating a step of moving carriers inthe material according to the present invention;

[0022]FIG. 5 is a fragmentary cross section diagram illustrating a waferhaving a current conducting therethrough via probes on the wafer surfaceand a charge detector device on the wafer surface according to thepresent invention;

[0023]FIGS. 6a-6 d are cross section diagrams of a semiconductormaterial illustrating charge carrier transport through the materialaccording to the present invention;

[0024]FIG. 7 is a fragmentary cross section diagram illustrating theHall effect and how an electron traveling through a material in thepresence of a magnetic field which is perpendicular to the currentexerts a force on the charged particle;

[0025]FIG. 8 is a block diagram of a charge detection system accordingto the present invention;

[0026]FIG. 9 is a flow chart diagram illustrating a method for detectingthe accumulated charge profile on a surface of the material by scanningthe surface and detecting the charge at each point according to thepresent invention; and

[0027]FIG. 10 is a fragmentary perspective view of the waferillustrating the scanning of the surface using a charge detector todetermine the doping concentration at each point on the wafer surfaceaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The following is a detailed description of the present inventionmade in conjunction with the attached Figures, wherein like referencenumerals will refer to like elements throughout. The present inventionrelates to a method of determining the doping concentration uniformityand the doping concentration of a semiconductor material such as alightly doped wafer used as a semiconductor starting material. Themethod includes moving carriers in the material, wherein the number ofcarriers is a function of the doping concentration of the material. Thecarriers are then deflected toward the surface of the material to forman accumulated charge profile on the material surface which is detectedusing a charge detector. The charge profile depends on the number ofdeflected carriers and thus reflects the doping concentration and thedoping concentration uniformity across the material.

[0029] In a preferred embodiment of the present invention, the carriersare moved by forcing a current through the wafer using probes. Theprobes make contact with the wafer surface and are connected to acurrent source which forces a constant current between the probes. Thecarriers are then preferably deflected using the Hall effect bysubjecting the wafer to a magnetic field which is perpendicular to thedirection of current flow. The magnetic field exerts a force on thecarriers which deflects the carriers toward the wafer surface, thuscreating a charge accumulation profile at the wafer surface. Theaccumulated charge is then detected by a charge probe such as an atomicforce microscope and used to calculate the doping concentration at eachpoint on the surface of the wafer.

[0030] Turning now to the Figures, FIG. 3 is a flow chart representing amethod 100 for determining the doping concentration of a semiconductormaterial. According to a preferred embodiment of the present invention,the semiconductor material is a lightly doped p-type semiconductor wafer101 to be used as starting material in a semiconductor manufacturingprocess. Alternatively, however, the method of the present invention isalso applicable to any type of doped semiconductor region or film andeach such film is contemplated as falling within the scope of thepresent invention.

[0031] At step 102 of the method 100, carriers are moved in the wafer.The moving of carriers in the wafer may be accomplished through avariety of means, as is well known by those skilled in the art.Preferably, the movement of carriers in the wafer occurs at step 102 byforcing a current through the wafer, as illustrated in the flow chart ofFIG. 4. Forcing a current begins at step 104 and includes applyingprobes 106 and 108 to a wafer surface 110, as illustrated in FIG. 5. Aconstant current is then forced through the probes at step 112 using,for example, a current source 114, as illustrated in FIG. 5.

[0032] Because the wafer 110 is lightly doped with a p-type dopant, thewafer is somewhat conductive (i.e., having a resistivity between that ofa conductor and an insulator; a semiconductor) and a current (i) flowsthrough the wafer 110 according to Ohm's law. The current (i) throughthe wafer is effectuated by carrier transport, as is well known by thoseskilled in the art. An exemplary model of carrier transport isillustrated in FIGS. 6a-6 d. In FIG. 6a, a p-type semiconductor material115 such as silicon includes a plurality of positively charge ions 116(in a p-doped material, an excess of positively charged ions existcompared to the number of free electrons in the material). The p-typematerial 115 also includes free electrons 118 that join with thepositive ions 116 to provide an electrically neutral atom. When avoltage (V) is supplied across the material 115, an electron 118 ispulled out at the left end 120 of the material 115, leaving the array ofatoms with a “hole” (i.e., a missing electron which is sometime called avacancy), as illustrated in FIG. 6a. The electrons 118 thenincrementally shift to the left to fill the hole in sequence, asillustrated in FIGS. 6a-6 d, thus resulting in the “hole” (i.e., thecarrier) moving through the material 115. The collective motion of theelectrons from right to left can thus be characterized as the motion ofa “hole” from left to right. As can be seen from FIGS. 6a-6 d, thenumber of carriers is a function of the doping level concentration inthe material 115. This relation may then be used to help determine thedoping level concentration in the material 115.

[0033] Similarly, the same principles may be used to characterizecarrier transport in n-type doped semiconductor materials. In such acase, the material has an excess of free electrons (as opposed to anexcess of positively charged ions) and current is provided through thetransport of negative carriers (i.e., the electrons). In any case, oncethe carriers have been generated at step 102, the carriers are thendeflected toward the wafer surface 110 at step 130 of FIG. 3.

[0034] According to a preferred embodiment of the invention, the carrierdeflection is accomplished by subjecting the wafer 101 to a magneticfield 132, as illustrated in FIG. 5. The magnetic field 132 is orientedin a direction perpendicular to the direction of current flow anddeflects the carriers toward the wafer surface 110 in accordance withthe Hall effect which will now be discussed and explained in conjunctionwith FIG. 7.

[0035] The force exerted on a particle in the presence of an electricand magnetic field is called the Lorentz force and is represented by:

F=q(E+υ×B),

[0036] wherein E is the electric field vector, B is the magnetic fieldvector and υ is the particle velocity vector (which in the case of thepresent invention is in a direction opposite to the direction of currentflow). As can be seen by the above equation, the force exerted by themagnetic field component is in a direction perpendicular to the flow ofcurrent if the magnetic field is oriented in a direction perpendicularto the direction of current flow since the magnetic force component(qυ×B) is determined by the “curl” of the velocity vector and themagnetic field vector. As illustrated in FIG. 7, if the current (i) isin the positive “x” direction (and thus the velocity vector is in thenegative “x” direction) and the magnetic field is directedperpendicularly out of the page, then, according to the “right handrule”, the force exerted on the particle due to the magnetic field willbe in the positive “z” direction, thus deflecting the particle towardthe wafer surface 110, as illustrated in FIG. 7.

[0037] The Hall effect employs this particle deflection principle. Ingeneral, the Hall effect can be summarized as follows: a conductivematerial having a current flowing in the presence of a perpendicularmagnetic field will result in a potential difference developing acrossthe conductor. This can be easily seen in FIG. 7. When the magneticfield 132 deflects a carrier (whether positive or negative) towards thewafer surface 110, an accumulation of charge develops at that point onthe wafer surface 110 which is proportional to the number of carriers inthe wafer at that point. The accumulation of charge thus results in thedevelopment of a voltage potential (V_(Z)) across the wafer, which isreferred to as the Hall voltage.

[0038] Once the carriers have been deflected toward the surface at step130, thus causing a charge accumulation profile to form on the wafersurface 110, the charge profile is detected at step 140 of FIG. 3. Inone exemplary embodiment of the present invention, the charge profilecan be detected using a voltage probe, a voltage meter or the like.Preferably, however, the charge profile is detected using an atomicforce microscope (AFM) such as a Dimension™ 5000 Scanning ProbeMicroscope manufactured by Digital Instruments. Using an AFM fordetecting the charge profile is preferred because the tip is extremelysmall which allows for an extremely high resolution charge profile to bedetected.

[0039] As is well known by those skilled in the art, an AFM 150, asillustrated in FIG. 8, includes a microminiature cantilever arm 151 anda sharp tip 152 which makes contact with the wafer surface 110. Thewafer 110 is placed on a scanning stage 153 which operates to move thewafer 110 with respect to the probe tip 152 to effectively scan thewafer surface 110. The stage 153 may incorporate traditional movement oractuation means (not shown) and may also include piezoelectric actuatorsfor effectuating high precision movement of the stage 153 to providehigh resolution scanning. Alternatively, the stage 153 may be keptstationary while similar actuation means may be used to move thecantilever arm 151/probe tip 152 assembly. Regardless of which componentis moved relative to the other, the AFM scanning mechanism 150 iscontrolled by a programmable controller 154 which is operable to receiveand analyze detected characteristics such as the accumulated charge anddisplay the collected information to the user via a display 155 andstore the data in a memory 156. Although the detection system 150 ofFIG. 8 does not illustrate a magnetic field means for the sake ofsimplicity, the magnetic field 132 of FIG. 5 may be provided using, forexample, a permanent magnet, a solenoid, coils, etc. as is well known bythose skilled in the art.

[0040] The AFM tip 152 preferably detects the accumulated charge in thefollowing manner. As opposed to a contacting mode of operation in whichthe probe tip 152 rides on the surface 110 of the material to bemeasured to profile the surface topology, the AFM 150 is operated in anon-contacting mode. In the non-contacting mode, the AFM tip 152 is helda short distance from the wafer surface 110 (about 5 Angstroms to 500Angstroms) and the tip 152 is deflected by electrostatic forces exertedagainst the tip 152 by the accumulated charge on the wafer surface 110.According to one exemplary embodiment of the present invention, theaccumulated charge at a point on the surface 110 is proportional to theelectrostatic force exerted on the AFM tip 152 which thus proportionallyimpacts the amount of deflection of the AFM tip 152. The amount ofcantilever arm 151 deflection is preferably measured using preciselyaligned optical components and a deflection measurement circuit (showncollectively as a cantilever arm detector 157 in FIG. 8), although othertechniques may be used and are contemplated as falling within the scopeof the present invention. For example, the cantilever arm may resonateat a frequency that may be varied in response to the influence of theelectrostatic forces caused by the accumulated charge.

[0041] An exemplary method 140 of detecting the charge profile includesscanning the wafer surface 110 with a charge detector 161 such as anAFM, as illustrated in the flow chart of FIG. 9, which will be discussedin conjunction with FIG. 10. Under the control of a microcontroller, thedetector 161 initializes the scanning coordinates at step 162 so thatthe detection scheme begins at a first coordinate (X₁, Y₁). Although notshown in FIG. 10 for the sake of simplicity, the probes 106 and 108(shown in FIG. 5) are placed across a square of the grid whichcorresponds to the location of the detector 161 so that each square ofthe grid will experience a uniform, constant current in the region beingdetected. Preferably, the first coordinate (X₁, Y₁) is at a corner of asquare grid 164, as illustrated in FIG. 10, and the grid 164 is scannedby the detector 161 row by row. Alternatively, any method of scanningsome or all of the points on the grid 164 may be used and each iscontemplated as falling within the scope of the present invention.

[0042] Once the detector 161 is at the coordinate (X₁, Y₁), the detector161 detects the charge amount at that point at step 166 by detecting thevoltage at that point by, for example, detecting the amount ofdeflection of the AFM tip 152 due to the electrostatic force exerted bythe accumulated charge as discussed above. After the charge amount isdetected at step 166, the detector 161 is moved to the next coordinate(e.g., X₂, Y₁) at step 168 and the charge amount at that point isdetected. The detector 161 continues scanning and detecting in the “x”direction until the “x” coordinate scan is complete at step 170 (YES),which indicates that the detector 161 has completed a row scan. Thedetector 161 then moves its position incrementally in the “y” directionat step 172 to start scanning a new row (e.g., X_(i), Y₂). The method160 checks to see whether the grid scan is complete at step 174 bychecking whether all the rows have been scanned. According to thepresent example, all the rows have not yet been scanned (NO), and themethod 140 returns to step 166 to scan the row in the “x” direction.Once all the rows have been scanned (YES at step 174), the method 140for detecting the charge profile on the wafer surface 110 is completedat step 176.

[0043] Returning now to FIG. 3, once the charge profile on the wafersurface 110 has been detected at step 140, the method 100 uses thedetected charge profile to determine the doping level concentration atone or more of the detected points at step 180. According to the presentinvention, the doping level is calculated using the detected chargeprofile (i. e., the measured Hall voltages) in the following mannerusing the variable references of FIG. 7.

[0044] To determine the carrier concentration (n) at a point (Y_(j),Y_(j)) on the wafer surface 110, the following relationship is used:

E_(Z)= V_(X)B_(Y),

[0045] which represents the equilibrium condition after the magneticfield is applied, wherein E_(Z) is the electric field formed by thecharge accumulation due to the magnetic field, B_(Y) is the appliedmagnetic field strength, and V_(X) is the carrier velocity which isoften called the Hall coefficient (R_(H)) and is equal to Jn/qn. Withrespect to the Hall coefficient, R_(H)=Jn/qn, Jn is the current (i), qis the electric charge and n is the carrier concentration. Since thedetected Hall voltage V_(Z) is detected and is directly related to theresultant electric field E_(Z), the carrier concentration (n) isinversely proportional to the detected Hall voltage and thus can beeasily determined.

[0046] In the above manner, the detected charge at each point on thewafer surface 110 can be used to determine the doping levelconcentration within the wafer 101. The detected charge profile may alsobe used to ascertain the doping concentration uniformity across thewafer. Furthermore, using an AFM to detect the charge profile isadvantageous since a high resolution detection scan may be performed.Consequently, the present invention provides a more accurate method ofdetermining the doping level of a semiconductor material with greaterresolution and lower cost than the prior art.

[0047] In addition to detecting the doping level concentration via theHall coefficient as discussed above, a simplified analysis may beconducted to merely evaluate the doping level uniformity using thedetected charge profile which was saved in the memory 156 of the AFMsystem 150. As discussed above in conjunction with FIG. 8, when thedetected charge at each point is detected by the AFM 150, the controller154 saves the charge data in the memory 156 (e.g., the charge at (X_(i),Y_(i))= Q(X_(i), Y_(i))). To evaluate whether the uniformity across thewafer surface 110 meets a minimum requirement, the controller 154 may,for example, access the data in the memory and insert the data into auniformity algorithm such:

Q(X _(i) , Y _(i))−Q(X _(i+1) , Y _(i+1))> Q _(threshold).

[0048] If the charge differential between incremental points is greaterthan a predetermined threshold, the controller 154 will then indicatethe nonuniformity to the user via the display 155. Alternatively, thecontroller may identify Q_(max) and Q_(min) for the wafer, determinetheir difference and compare the difference to a threshold foruniformity purposes. In the above manner, without performing additionalcalculations, the system 150 may provide information on whether thedoping concentration level across the wafer is sufficiently uniform.

[0049] Although the invention has been shown and described with respectto a certain preferred embodiment or embodiments, it is obvious thatequivalent alterations and modifications will occur to others skilled inthe art upon the reading and understanding of this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described components (assemblies, devices,circuits, etc.), the terms (including a reference to a “means”) used todescribe such components are intended to correspond, unless otherwiseindicated, to any component which performs the specified function of thedescribed component (i.e., that is functionally equivalent), even thoughnot structurally equivalent to the disclosed structure which performsthe function in the herein illustrated exemplary embodiments of theinvention. In addition, while a particular feature of the invention mayhave been disclosed with respect to only one of several embodiments,such feature may be combined with one or more other features of theother embodiments as may be desired and advantageous for any given orparticular application.

What is claimed is:
 1. A method of determining a doping concentration ofa semiconductor material, comprising the steps of: moving carriers inthe material, wherein a number of carriers is a function of the dopingconcentration of the material; deflecting the carriers toward a surfaceof the material; and detecting an accumulated charge on the surface ofthe material due to the deflected carriers.
 2. The method of claim 1 ,wherein moving carriers comprises forcing a current in the material. 3.The method of claim 2 , wherein forcing a current in the materialcomprises: placing a plurality of probes on a surface of the material;and applying a current source across at least two of the plurality ofleads.
 4. The method of claim 1 , wherein deflecting the carrierscomprises subjecting the material to a magnetic field.
 5. The method ofclaim 4 , wherein a direction of the magnetic field is perpendicular toa direction of flow of the carriers, wherein the magnetic field exerts aforce on the carriers due to the Hall effect.
 6. The method of claim 1 ,further comprising calculating a doping concentration at a point on thematerial surface using the detected accumulated charge at the point onthe material surface.
 7. The method of claim 6 , further comprising:scanning the surface of the material; and calculating the dopingconcentration at each point on the scanned material surface using thedetected accumulated charge at each respective point on the materialsurface.
 8. The method of claim 1 , wherein detecting the accumulatedcharge on the material surface comprises: placing a probe at a point onthe material surface; and measuring a voltage at the point.
 9. Themethod of claim 8 , wherein the probe is an atomic force microscope tip.10. The method of claim 9 , wherein measuring the voltage with the tipcomprises detecting an amount of deflection of the tip due to anelectrostatic force exerted on the tip by the accumulated charge. 11.The method of claim 1 , wherein detecting an accumulated chargecomprises: (a) initializing a scanning coordinate of a detectionapparatus; (b) detecting a charge amount at a first point on thematerial surface; (c) moving the detection apparatus to another point onthe material surface; (d) detecting the charge amount at the otherpoint; and (e) repeating steps (c) and (d) until the surface of thematerial is substantially scanned and an accumulated charge profile isdetected.
 12. The method of claim 1 , wherein detecting an accumulatedcharge comprises: (a) initializing a scanning coordinate for a chargedetector; (b) scanning a first row in a first direction and detectingthe accumulated charge at a plurality of points in the first row withthe charge detector; (c) moving the charge detector in a directionperpendicular to the first row to a next row; (d) scanning the next rowin the first direction and detecting the accumulated charge at aplurality of points in the next row with the charge detector; (e)repeating steps (c) and (d) until the surface of the material issubstantially scanned and an accumulated charge profile is detected. 13.The method of claim 1 , wherein detecting an accumulated chargecomprises: (a) initializing a scanning coordinate for an atomic forcemicroscope tip; (b) placing the atomic force microscope tip at theinitialized coordinate; (c) detecting a charge amount at the initializedpoint on the material surface; (d) moving the atomic microscope tip in afirst direction to another point on the material surface; (e) detectingthe charge amount at the other point; (f) repeating steps (d) and (e)until the atomic force microscope tip has scanned a predetermined numberof points in the first direction, thereby scanning a row; (g) moving thetip in a second direction substantially perpendicular to the firstdirection to another row; (h) detecting a charge amount at the point ofthe atomic force microscope tip location; (i) repeating steps (d) and(e) until the atomic force microscope tip has scanned a predeterminednumber of points in the first direction, thereby scanning another row;and (j) repeating steps (g), (h) and (i) until the atomic forcemicroscope has scanned substantially the entire surface of the material.14. A system for determining a doping concentration uniformity of asemiconductor wafer, comprising: a stage accommodating a wafer to beevaluated; an atomic force microscope system operatively coupled to thestage; magnetic field means associated with the stage for subjecting thewafer to a magnetic field and deflecting carriers in the wafer to asurface of the wafer, thereby generating an accumulated charge profile;and a controller operatively coupled to the stage and the atomic forcemicroscope for dictating the detection of the accumulated charge profileon the wafer and controlling a positional relationship between the stageand the atomic force microscope.
 15. The system of claim 14 , whereinthe atomic force microscope comprises a microminiature cantilever armand an atomic force microscope tip for detecting the accumulated chargeprofile on the wafer.
 16. The system of claim 15 , wherein the atomicforce microscope operates in a non-contacting mode and detects theaccumulated charge profile by sensing a deflection of the cantilever armand tip due to electrostatic forces exerted by the charge profile on thetip.
 17. The system of claim 16 , wherein the atomic force microscopefurther comprises a cantilever arm detector which senses the deflectionof the cantilever arm and converts the sensed deflection to a detectedcharge value.
 18. The system of claim 14 , wherein the controller savesthe detected accumulated charge profile as data in a memory, and usesthe data to determine whether the doping concentration of the wafersatisfies a uniformity criteria.
 19. The system of claim 14 , whereinthe controller saves the detected accumulated charge profile as data ina memory and uses the data to calculate the doping concentration acrossthe wafer using an algorithm based on the Hall effect.